1. Field of Invention
The present invention relates generally to an apparatus, and method for conducting measurements in or via a subsurface borehole. More particularly, the invention relates to such an apparatus and method for addressing certain “borehole eccentricity effects” encountered in such subsurface measurements, and more specifically, to reducing and/or correcting these borehole effects.
2. Background Art
Various resistivity logging techniques are employed in hydrocarbon exploration and production operations, including galvanic techniques (e.g., laterologs) and electromagnetic (EM) induction techniques. Both of these techniques employ logging instruments or “sondes” to emit energy (current or EM field) into the formation or environment surrounding a subsurface borehole. The emitted energy interacts with the formation to produce response signals that are detected by sensors on the instrument. The detected signals are then processed to establish a profile of one or more properties of the formation.
To ensure high quality measurements, the well logging tool is preferably maintained at or near the center of the borehole (i.e., along the longitudinal axis). It can be difficult, however, to maintain centering of the tool at all times. As the tool deviates from the center of the borehole toward the borehole wall, an otherwise accurate or desirable response signal may change (although the characteristics of the formation being measured has not). This change in the signal is referred to as the “standoff effect” or “eccentering effect” (hereforth, “borehole eccentricity effects”). An eccentered induction tool can, for example, induce very strong borehole-produced signals that interfere with the response signals from the formation.
The extent of the signal change due to the borehole eccentricity effect varies depending on the type of tool conducting the measurement. In the case of a resistivity tool, the signal may be influenced by changes in the location of the tool in the borehole and the resistivity of the drilling mud. The present invention relates generally to an apparatus and method for addressing any of these undesirable effects, particularly those arising from borehole currents, and to all types of resistivity logging, including electromagnetic (EM) induction logging.
Conventional wireline EM logging instruments are implemented with antennas that function as sources and/or sensors. On wireline EM logging instruments, the antennas are typically enclosed by a housing constructed of a tough plastic (insulating) material, e.g., a laminated fiberglass material impregnated with epoxy resin. Alternatively, these instruments may be constructed of thermoplastic (insulating) materials. The thermoplastic material of these instruments provides a non-conductive structure for mounting the antennas. U.S. Pat. No. 6,084,052 (assigned to the present assignee) discloses a composite-based logging instrument for use in wireline and LWD applications, as contemplated by the present invention.
The antennas are typically spaced apart from each other along the axis of the tool. These antennas are generally coils of the solenoid type comprising one or more turns of insulated conductor wire wound around a support. U.S. Pat. Nos. 4,651,101, 4,873,488, and 5,235,285 (each assigned to the present assignee), for example, disclose instruments equipped with antennas disposed along a central metallic support (each hereby incorporated by reference and made a part of the present disclosure). In operation, the transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (also referred to as mud) and into the formation. The signals detected at the receiver antenna are usually expressed as a complex number (phasor voltage) and reflect interactions of the emitted energy with the mud and the formation.
A coil (or antenna) carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area. The direction and magnitude of the magnetic moment is represented by a vector perpendicular to the plane of the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their magnetic dipoles aligned with the longitudinal axis of the instruments. Such instruments are, therefore, referred to as having longitudinal magnetic dipoles (LMD). When an LMD tool is placed in a borehole and energized to transmit EM energy, the induced eddy currents flow in loops around the antenna in the borehole and in the surrounding formation. These eddy currents flow on planes that are perpendicular to the tool's longitudinal axis (which corresponds with the borehole axis) but do not flow up or down the borehole.
An emerging technique in the field of EM induction well logging is the use of instruments incorporating antennas that have tilted or transverse antennas. The magnetic dipoles of these antennas are tilted relative to or perpendicular to the tool axis. Such instruments are referred to as having transverse or tilted magnetic dipoles (TMD). These TMD instruments can, therefore, induce eddy currents that flow on planes that are not perpendicular to the borehole axis. As a result, these TMD tools provide measurements that are sensitive to dipping planes, formation fractures, or formation anisotropy. Logging instruments equipped with TMDs are described, for example, in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, and 6,147,496 (each of which is hereby incorporated by reference and made a part of the present disclosure).
While TMD tools are capable of providing improved formation resistivity measurements, these tools tend to be significantly influenced by borehole currents. This is particularly true in high contrast situations, wherein the mud in the borehole is more conductive than the formation. When a TMD tool is energized at the center of the borehole, it can induce eddy currents that flow up and down the borehole. However, due to the symmetry in current flow, the up and down currents cancel each other, thereby providing zero net current flow in the axial or longitudinal direction. When a TMD tool is eccentered, however, there may not be any such symmetry in the current flow. If the TMD tool is eccentered in a direction parallel to the direction of the magnetic dipole of its antenna (i.e., longitudinal eccentricity) the symmetry plane that includes the borehole axis and direction of dipole moment is maintained and thus there is zero net current flow along the longitudinal or borehole axis. However, if a TMD is eccentered in a direction perpendicular to the direction of the magnetic dipole of its antenna (called transverse eccentricity), there is no such symmetry. Accordingly, there is a resultant current flow up or down the borehole, (when the antenna is energized). In high contrast situations (i.e., conductive mud and resistive formation), the borehole currents can flow a long distance along the borehole. When these currents pass in the vicinity of TMD receivers, they induce undesired signals that can be much larger than the actual response signals from the formation.
Some of these undesirable effects (signals) may be attenuated during data processing. For example, U.S. Pat. No. 5,041,975 (assigned to the present assignee) discloses a technique for processing data from downhole measurements to correct for borehole effects. U.S. Pat. No. 6,541,979 (assigned to the present assignee) discloses techniques for reducing the effect of borehole eccentricity, using mathematical corrections for the borehole currents effects.
Alternatively, the undesirable effects from borehole currents may be minimized during data acquisition. For example, U.S. Pat. No. 6,573,722 (assigned to the present assignee) discloses methods to minimize the borehole currents passing TMD antennas. In one method, an electrode located below the TMD antenna is connected to another electrode located above the TMD antenna to provide a conductive path beneath the TMD antenna. This additional conductive path reduces the amount of borehole currents passing in front of the TMD antenna, and thus minimizes the undesirable effects. In another method, a tool is disclosed that generates a localized current in the borehole (between the two electrodes located on either side of a TMD antenna) that counteracts or cancels out the undesirable borehole currents. However, the localized current itself has an adverse effect on the TMD antenna, albeit to a lesser extent than the borehole currents.
While these prior art methods and tools provide means to reduce the effects of borehole currents, there remains a need for further improvements in the development of systems, methods, and apparatus to reduce, eliminate, or otherwise address the undesired effects of borehole currents.